It would be nice. There's an older package for doing Java completion, JDEE, which takes a similar approach (running a helper in BeanShell). But it is difficult to configure, and fragile, and doesn't have enough active developers to keep it up to date.

OmniSharp is a lot simpler, in that you just point the server at a solution file, and it just works. There was talk on the JDEE mailing list a while back about doing something similar using Maven POMs, but I don't think it took off.